1. Field of the Invention
The present invention relates to refrigerator oil compositions, and specifically to refrigerator oil compositions which have excellent lubricating property and are suitable for use in compression refrigerators operated using a fluoroalkane refrigerant.
2. Description of the Related Art
Refrigerator oils composed of a base oil such as a mineral oil, alkyl benzene, polyglycol or a mixture thereof and optionally added with an extreme pressure additive have heretofore been used widely in compression refrigerators which employ CFC-11, CFC-12, CFC-115, HCFC-22 or the like as a chlorine-containing refrigerant.
Among these refrigerants, CFC types including CFC-11, CFC-12 and CFC-115 are subjected to control as they are considered to lead to destruction of the ozone layer. For similar reasons, there is also a move toward controlling hydrogen-containing chlorinated hydrocarbon refrigerants such as HCFC-22. Fluoroalkane refrigerants, particularly HFC-32, HFC-125, HFC-134a and HFC-152a are similar in thermodynamic properties to CFC-12 and HCFC-22 so that they are under investigation or are being used as substitutes for Cl-containing refrigerants.
A refrigerator lubricating oil is required to satisfy a variety of properties. Of these, lubricating property is extremely important from the viewpoint of the reliability of a refrigerator system.
As lubricating oils for refrigerators cooled using a Cl-containing refrigerant such as CHF-12 or HCFC-22, there have been known lubricating oils containing, in addition to a base oil such as a mineral oil, alkylbenzene or a mixture thereof, a phosphate ester and/or a phosphite ester (Japanese Patent Laid-Open No. 91502/1979), trioleyl phosphate (Japanese Patent Laid-Open No. 86506/1976), a phosphite ester (Japanese Patent Laid-Open No. 139608/1979), tricresyl phosphate and/or triphenyl phosphite (Japanese Patent Laid-Open No. 27372/1980), a phosphate ester and a hydrogen phosphite ester (Japanese Patent Laid-Open No. 92799/1980), an organomolybdenum compound and an acid phosphate ester (Japanese Patent Laid-Open No. 75995/1984) or a thiophosphate (Japanese Patent Laid-Open No. 293286/1986).
The lubricating oils containing a phosphite ester therein, however, have the drawback that the phosphite ester reacts with water which remaining in or penetrated in a refrigerator system and forms phosphoric acid, thereby corroding metals in the system.
The lubricating oils containing a thiophosphate therein also have the drawback that thermal decomposition products of the thiophosphate corrode copper pipes in a system, windings of a motor in a hermetic-type compressor, and the like.
Chlorine atoms contained in a refrigerant molecule in a large amount in the system act as an extreme pressure additive, as have already been reported by Honma et al. in the Preprint D.9 (1989) of the 34-th National Meeting of Japan Society of Lubrication Engineers. When such conventional additives are employed in combination with a Cl-containing refrigerant such as CFC-11, CFC-12, CFC-115 or HCFC-22, their function as extreme pressure additives are not particularly important. The addition of a phosphate ester, phosphite ester, acid phosphate ester or hydrogen phosphite ester alone is sufficient.
Fluoroalkane refrigerants containing no chlorine atom or atoms in their molecules, such as HFC-32, HFC-125, HFC-134a and HFC-152a, however, have no effects as an extreme pressure additive so that the addition of an extreme pressure additive is indispensable for a lubricating oil employed in a compression refrigerators using a fluoroalkane as a refrigerant.
It is, on the other hand, important for a refrigerator oil to have good miscibility with a refrigerant. Refrigerator oils for a fluoroalkane refrigerant employ a base oil having strong polarity such as an ester oil or polyglycol oil in view of their miscibility with the refrigerant. In this case, however, strong polarity of the ester oil or polyglycol oil significantly deteriorates physical and chemical adsorption of an extreme pressure additive on a sliding metal surface, thereby lowering the effects of the extreme pressure additive so added. Further, each extreme pressure additive can act only within a certain specific temperature range so that the addition of only one extreme pressure additive is not fully effective for a commercial compressor which is operated in a wide temperature range.
U.S. Pat. No. 4,755,316 discloses a lubricating oil for refrigerators in which a fluoroalkane is used as a refrigerant. The lubricating oil comprises, as a base oil, a polyalkylene glycol and, as an extreme pressure additive, a phosphate ester, phosphite ester or thiophosphate ester. Although these extreme pressure additives have been known to date, their single use in refrigerator systems making use of a fluoroalkane as a refrigerant and a polyalkylene glycol as a lubricating oil does not allow the extreme pressure additives to fully exhibit their effects in commercial refrigerators operated in a wide temperature range no matter which extreme pressure additive is used.